It is known that, for example in nuclear reactors, the cover is fastened to the vessel removably, to make it possible to recharge the reactor periodically with fuel and, by means of this cover, to inspect the interior of the vessel and the elements located inside it.
For this purpose, the cover is fastened to the reactor vessel by means of pins which are screwed into the collar of the vessel and which penetrate smooth holes provided in the collar of the cover. These pins are usually used in fairly large numbers and are relatively close to one another, with the result that all these pins have to be unscrewed and screwed in order to remove and refit the cover.
Furthermore, during the tightening and loosening operations, a pull is exerted on the pins by means of a tension device, to put them in a prestressed state.
For this purpose, FR-A-No. 2,329,414 makes known a tension device composed of an assembly of hydraulic tensioners fastened and distributed in polar symmetry on a supporting ring. The hydraulic tensioners act simultaneously and exert the desired prestress on each pin. To exert the desired force, each hydraulic tensioner has several superimposed stages, each comprising a piston provided with a central bore, through which the pin shank passes, and having a lower part forming a guide skirt and an upper part of larger diameter forming a piston. This arrangement is moreover necessitated by the reduced space between the pins. Each stage has a cell provided with a shoulder surrounding the said piston and its skirt. There are means of introducing a fluid at high pressure at the said shoulder, and the upper stage interacts with a traction device fastened to the end of the pin. The thickness of the skirts of the said pistons increases from the lower stage to the upper stage.
This device therefore has several superimposed stages for each tensioner, thus resulting in a multiplicity of gaskets, causing difficult and costly maintenance and increasing the risks of leaks. Furthermore, because the overall size of the device depends on its environment, the force necessary for obtaining the prestress is limited by the usual working-fluid pressures, and, if the force is to be increased, this can only be done by increasing the number of superimposed pistons.
On the other hand, FR-A-No. 2,274,997 makes known a device for screwing and unscrewing under-tension for opening or closing the cover of a vessel of a nuclear reactor, with a supporting ring provided with lifting jacks which, under pressure, subject the vessel pins to tension. This device makes use of a pair of two single-stage jacks for each pin, thus increasing considerably the number of jacks on the supporting ring. Consequently, one of the difficulties with this device is the coordination of the simultaneous displacement of the pistons of all the jacks. Another disadvantage of such a device is that the power level remains limited because of its overall size.